Abstract: We report the theoretical and experimental demonstration of pattern formationin acoustics. The system is an acoustic resonator containing a viscous fluid.When the system is driven by an external periodic force, the ultrasonic fieldinside the cavity experiences different pattern-forming instabilities leadingto the emergence of periodic structures. The system is also shown to possessbistable regimes, in which localized states of the ultrasonic field develop.The thermal nonlinearity in the viscous fluid, together with thefar-from-equilibrium conditions, are is the responsible of the observedeffects.